1. Field of the Invention
The present invention relates to an over-running clutch equipped for, for example, a starter of an internal combustion engine of an automobile, or the like. More particularly, the invention relates to an over-running clutch structure which is improved in torque transmission characteristics and durability.
2. Description of the Related Art
FIG. 4 is a sectional view illustrative of a conventional over-running clutch, for example, disclosed in Japanese Utility Model Laid-Open No. 59-195231. FIG. 5 is a sectional view along line V--V of FIG. 4.
Referring to FIGS. 4 and 5, a thrust spline generally denoted by 2 is constructed to be opened toward a pinion 7 and comprises a boss portion 3 which is helically spline-engaged with an output shaft 1, a cam bottom 4, and a clutch outer 5, which components are formed integrally with each other. The clutch outer 5 has five grooves 5a which are formed circumferentially around its innner peripheral surface at an equal pitch, each having a wedge-like shape so that each gradually decreases around the circumference in a single direction and having an uniform internal diameter along the axis.
A clutch inner 6 is formed into a cylindrical shape having a uniform external diameter along the axis and is formed integrally with a pinion 7 used for transmitting power to a ring gear of an engine(not shown). The clutch inner 6 is placed inside the clutch outer 5 to form wedge-like spaces between the outer peripheral surface of the clutch inner 6 and the grooves 5a, and is attached rotatably to the output shaft 1.
Rollers 8 are accommodated in each of the wedge-like spaces so as to be circumferentially movable. Urging springs 9 are further accommodated in each of the wedge-like spaces so as to urge the rollers 8 into narrower portions of the wedge-like spaces. A washer 10 is fixed to the opened end of the clutch outer 5 by a clutch cover 11 so that it can restrict the axial movement of the rollers 8 and can also shield the wedge-like spaces.
A description will now be given of the operation of the conventional over-running clutch with reference to FIG. 6.
The torque of the output shaft 1 is transmitted to the thrust spline 2 via the boss portion 3 which is helically spline-engaged with the output shaft 1, thereby rotating the thrust spline 2. Then, the rollers 8 are wedged between the clutch outer 5 and the clutch inner 6, and the torque of the thrust spline 2 is transmitted to the clutch inner 6 via the rollers 8 so as to allow the pinion 7 to rotate. This further permits the torque of the pinion 7 to be transmitted to the ring gear of the engine.
Simultaneously, each roller 8 revolves into a narrower portion of the wedge-like space within each of the grooves 5a, thereby causing relative distortion between the clutch outer 5 and the clutch inner 6. Accordingly, a load proportional to a transmitted torque is distributed axially on the abutting surfaces of each of the clutch outer 5 and the clutch inner 6 with the rollers 8.
In general, cementation quenching is performed on the thrust spline 2 in order to improve its mechanical strength. However, since the thrust spline 2 is constructed to be opened toward the pinion 7, a thermal treatment performed during cementation quenching causes the thrust spline 2 to be deformed. Due to such thermal deformation of the thrust spline 2 the internal diameter of the clutch outer 5 adjacent to the cam bottom 4 is inclined to be smaller than the internal diameter adjacent to the opened end. This makes the overall clutch outer 5 slightly diverged toward the opened end. In contrast to the clutch outer 5, the clutch inner 6 is constructed such that the external diameter of the clutch inner 6 is formed uniformly along the axis. The clutch outer 5 thus has a disparity between the internal diameter adjacent to the cam bottom 4 and that adjacent to the opened end. Such a clutch outer 5 is combined with the clutch inner 6 having a uniform external diameter. This causes the rollers 8 to be skewed along the axis and fails to ensure uniform distribution of the pressure of the rollers 8 during the transmission of the torque.
An analysis was made according to a finite-element method so as to find a pressure acting on the clutch inner 6 during the transmission of the torque. As indicated by arrows A in FIG. 6, pressure produced at the rear end (adjacent to the cam bottom 4) of the clutch inner 6 is much greater than that produced at the end of the clutch inner 6 adjacent to the pinion 7 (adjacent to the opened end of the clutch outer 5).
Since the conventional over-running clutch is constructed as described above, the rollers 8 are skewed along the axis, thereby failing to ensure uniform distribution of the axial pressure of the rollers 8 during the transmission of the torque. Hence, the conventional over-running clutch presents the following problems. Good torque transmission cannot be achieved. Besides, the clutch inner 6 which is partially subjected to a heavy load are likely to be indented, and the rollers 8 which are also partially subjected to a heavy load are likely to be worn nonuniformly, thus resulting in a decrease in the durability of the over-running clutch.